1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method. More particularly, the present invention relates to correction for suppressing an uneven density caused by variations of print characteristics of printing elements, the correction being performed in an image processing system for performing quantization with a dither matrix.
2. Description of the Related Art
The head shading (abbreviated as “HS”) technique disclosed in Japanese Patent Application Laid-open No. Hei 10-13674 (1998) has been known as one example of the correction for suppressing an uneven density of the aforementioned type. This HS technique is adapted to perform correcting based on information on ink ejection characteristics (i.e., print characteristics) of each of nozzles serving as printing elements. For example, in response to information that amount of ink to be ejected by a certain nozzle is more than normal amount, image data is corrected so as to decrease a gradation value indicated by the image data corresponding to the nozzle. In contrast, in response to information that amount of ink to be ejected by a certain nozzle is less than the normal amount, image data is corrected so as to increase a gradation value indicated by the image data corresponding to the nozzle. In this manner, the number of ink dots to be eventually printed is increased or decreased, so that the density of a print image formed based on the image data can become substantially even in each of the nozzles.
When a patch is printed in order to create a correction parameter used for the HS or the like, the relationship between the number of pixels that is a unit for the correction and the arrangement of thresholds on the dither matrix may be often improper in the case of the use of a dither matrix for quantization for generating print data for the patch. More specifically, the number of pieces of data indicating printing a dot (“1” signifies printing; hereinafter it may be simply referred to as a “dot”), which is generated in a processing unit corresponding to a certain printing element, and the number of dots generated in a processing unit corresponding to another printing element, with respect to image data having the same gradation value may be remarkably different from the arrangement of the thresholds on the dither matrix. As a consequence, the density measured of a printed patch is different according to the printing elements in the processing unit, and therefore, the print characteristics of each of the printing elements in the processing unit cannot be accurately reflected. That is to say, even if the print characteristics of all of the printing elements are the same as each other, the measurement result of the patch printed based on quantization data with the dither matrix shows that the print characteristics of the printing elements are different according to the processing units. And thus, an uneven density occurs on a print image in accordance with the image data corrected with a correction parameter formed based on the measurement result.
In order to avoid the above-described problem that the number of dots to be formed is different in each of the processing units for the correction, it is conceived to form a dither matrix having the arrangement of thresholds in such a manner as to achieve the same number of dots in every processing unit. However, the condition of the formation of such a dither matrix restricts the arrangement of thresholds capable of achieving a target quality of an image, such as reduction of granularity, which is inherent to a dither matrix of a spiral type or Bayer type.